This invention relates to housing electrical assemblies and, more particularly, to methods and apparatus for housing electrical components to reduce electromagnetic and radio frequency interference.
Electrical assemblies, e.g., computers, are typically housed in cabinets providing physical support and protection. The time-variant electromagnetic fields generated by those assemblies readily propagate through conducting media and space, and have long plagued manufacturers. These fields, referred to as "EMI" (electromagnetic interference) and "RFI" (radio-frequency interference), can interfere with the other electrical equipment operating in the vicinity. Cabinets designed to shield this interference are known as "EMI cabinets."
The prior art has employed numerous structures for limiting EMI and RFI. U.S. Pat. No. 4,780,570 of Chuck, for example, suggests affixing a strip of conductive finger-like springs to EMI cabinet walls. When an electrical component is inserted into the cabinet, it engages the springs with a wiping contact.
U.S. Pat. No. 4,322,572 of Synder, suggests a similar mechanism for maintaining interference-suppressing electrical contact between a grooved support member, e.g., a cabinet frame, and a closure member, e.g., a side panel, that slides into the groove. The Synder patent discloses a shielding strip having cantilevered springs disposed along one face and tangs disposed along the other. The strip is positioned in the groove so that, as the closure member slides into the groove, it is wipingly engaged by the closure member and its tangs are forced into contact with the groove.
Because they rely on sliding motion, the Chuck and the Synder mechanisms are of little value in limiting interference between components that move into engagement in a direction normal to their surfaces. By way of example, mini- and main-frame computer components are traditionally bolt mounted to EMI racks. To shield these racks, the art has traditionally relied on a braided ground cable or wire mesh applied to the racks prior to installation of the component panels.
Though these techniques have met limited success, breaks in the cable or mesh seal resulting from oxidation, unremoved lacquers or other surface coatings, loss of adhesive strength, and the like can cause the shield to break down, thus permitting excess EMI and RFI. This interference becomes more problematic as operating speeds, e.g., clock speeds, of the component circuitry increases.
Other problems with prior EMI grounding contact devices is that they generally involve added components, which in turn require assembly with exacting electrical connection to other conductive members. Thus these prior practices tend to be costly and prone to failure.
Accordingly, an object of the invention is to provide improved methods and apparatus for housing electrical apparatus.
More particularly, an object is to provide an improved mechanism to attenuate EMI/RFI interference from rack-mounting electrical components.
A further object of the invention is to provide a low-cost and high reliability mechanism for providing EMI/RFI interference-limiting engagement between components which are brought into mounting abutment in a direction normal to their surfaces.